At one time, the issue of voice intelligibility in mass notification systems was an afterthought. In the early years of the technology, the goal was to simply notify the populace of a designated area that an emergency event was occurring and in the case of an air raid or fire, this was usually accomplished through the use of a loud siren.

As the times have changed, however, mass notification technology, including voice intelligibility; have had to change with them. Modern day case studies show that certain situations, such as an active-shooter scenario or severe storm, require the use of voice messages to direct people to take specific actions.

Though voice intelligibility was not a big focus for MNS manufacturers at one time, Mark Kurtzrock, president and CEO of mass notification systems manufacturer Metis Secure Solutions, says that more people in the industry now realize the benefits of having voice warnings that are easy to understand.

"Initially, I think it was not a factor that people had considered. But, as more and more products have come on the market and more regulations have been put into place... quite frankly it seemed like an obvious solution that people hadn't focused much attention on," Kurtzrock explained. "It's not just about sending one message. Circumstances change, events are very dynamic and the ability to communicate clear information to people on an ongoing basis is really important. "

For example, Kurtzrock said that if an organization's corporate campus consisted of a research facility and several other office buildings, there could be an incident that forces the evacuation of the research building, but people in the other offices would need to stay put.

"There are a lot of systems that don't have a voice component at all. If you look at sirens or cellphone text messaging, where it's just a matter of a message coming across, it doesn't necessarily stop a person from what they're already doing, especially if they don't understand it," said Amy Baker, director of marketing at Metis Secure Solutions. "For example, if a siren is used for several different things, how do you know what that tone really means to you? How many people are really trained on what a tone means and do they know what to do when it occurs? Inside buildings, we've seen systems that don't have a broadcast voice and you can't reach as many people and you can't get into classrooms or conference rooms where people may have cellphones turned off."

Robert (Bob) Lang, assistant vice president for strategic security and safety at Kennesaw State University in Georgia, says their campus uses a voice siren system that can broadcast up to eight different messages. Lang said that they've only had to actually use five of these messages, which included warnings about tornadoes and an escaped prisoner in the area. The warnings and what they include are posted on the KSU website.

Initially, Lang said that they experienced difficulties with the intelligibility of warning messages from the siren system because they sounded like a "factory voice" or too much like computer text-to-speech translations. KSU found a simple solution for the problem.

"We had our baseball announcer record the messages for us," Lang said. "We believe they're more intelligible now and people can understand what is being said."

While some systems provide users with the ability to send out live voice messages, in most cases Kurtzrock said using canned, pre-recorded messages is the better option.

"When an event is going on, human nature is always a factor in any situation," he explained. "If people can simply go to a menu, select that message and shoot it out through the system, you're going to be assured that people are going to hear what they need to hear in the right time frame, as opposed to having to instruct and come up with that information on the fly."

Of course, there are many factors that can impact the intelligibility of voice warning system such as the acoustical properties of a building and speaker placement.

"Through a lot of field testing, we know that our alerting devices need to have a certain distance between them," Kurtzrock said. "A typical alarm system is at say 90 dB (decibels). Our voice information is generally at around 75 dB and the reason for that is that the clarity... is very, very self-evident. We also use very sophisticated sound compression for the actual recording of the messages so that the clarity of the information coming out is predetermined. We know exactly how it will sound before the system goes live."

Kurtzrock said that the architecture of a building also plays a significant role in voice intelligibility.

"In some cases you have narrow hallways; you can also have tile floors as opposed to carpet. Carpeting picks up sound and it dulls the sound a bit, so you have to do a fair amount of testing in terms of the distribution," Kurtzrock said of his company's voice systems. "At a lot of places they just install loudspeakers in a hallway; one speaker in one hallway and another in the next and just assume that that's good enough."

Helping to establish standards for voice intelligibility, industry groups like the National Fire Protection Association (NFPA) are now addressing the technology in their building codes.

"NFPA 72 requires that Intelligibility be measured for both fire and MNS systems that provide voice signaling," said Derek Mathews, senior staff engineer, life safety & security at Underwriters Laboratories. "Annex D provides some guidance on the planning, design, installation, and testing of voice communication systems. Intelligibility is not being mandated by all authorities so some manufacturers have had to address it and some have not."

According to Mathews, the issue of voice intelligibility has been a part of code discussions even before mass notification systems were brought under the umbrella of life safety systems.

"Intelligibility was being discussed before MNS entered the life safety realm (NFPA 72) because fire alarm systems can also provide voice communication for either evacuation or relocation purposes," he explained. "Speech intelligibility is the same for both MNS and fire systems. Lots of research and studies have been conducted in order to fully understand Intelligibility. Currently, there is guidance in Annex D in NFPA 72 2010, for speech intelligibility."

As a manufacturer of speakers for mass notification systems, Christa Poss, marketing manager for the audible/visible business unit at System Sensor, says that they've focused their efforts on improving the quality of their speakers and providing installers with tools to help them optimize intelligibility.

"I think previously, there were so many unknowns about intelligibility and now there is just a lot more talk about it, people are getting educated on it and now it's a real issue because it's in the code," Poss said.

According to Poss, one tool that has been in use by the pro audio market since the 90s that can help voice notification system designers is a speaker modeling software called EASE from German-based ASMG Technologies.

"They've made a version specifically scaled for the life safety market. They took out a lot of the advanced functionality that we just wouldn't use in this market so a fire alarm designer can easily input the model of a room, put the acoustical properties of the space... where the speakers are placed and it's going to give you a prediction of your audibility and intelligibility," she said.

Mike Flannery, director of commercial hardware for ADT, says that from a design standpoint on interior systems, his company generally always tries to use a large number of speakers at lower volumes.

"In an interior space, with a fire alarm system, you're just trying to get the person's attention with a very loud blast of noise or voice, give them some very basic instructions, turn strobes on, etc., but with mass notification, the instructions may be more detailed," he explained.

For exterior spaces, Flannery said that there are a combination of a couple of different technologies that can be utilized, including traditional giant voice speakers that are loud and project over long distances and planar speakers, which are fairly directional, but much more intelligible than traditional speakers. He emphasized, however, that organizations that are implementing voice systems need to make sure that they have people with the proper skill sets installing them to ensure audibility and intelligibility.

"You should always have people that have these special skills, sound design engineers, people with NICET training to do these sound designs because they really are a specialized skill set," he said.